The referenced application Ser. No. 06/319,655, filed Nov. 9, 1981, of which the inventor hereof is a co-inventor, describes an FM receiver which has a frequency demodulator for the carrier signals and the audio intelligence information modulated thereon, as well as a detector for a 57 kHz auxiliary carrier which is amplitude modulated, the receiver including an amplitude demodulator for the amplitude modulation of the 57 kHz subcarrier. The additional filtering circuit and demodulation circuit for the amplitude demodulation of the 57 kHz subcarrier is utilized to recognize the subcarrier which is radiated by some radio stations, capable of providing special announcements, for example of traffic conditions, sports programs, or the like. For details, the referenced Hegeler patent, and the applications are referred to. The modulation on the subcarrier can provide recognition of a predetermined radio station, or region, hereinafter RR signals, which are separate from the audio program being provided by the station on the main carrier. In the time during which a special announcement is being radiated, the auxiliary 57 kHz subcarrier can be modulated by a different, or additional announcement recognition frequency, hereinafter the AR signal. The transmitter, thus, by radiating suitable signals can control switching within the receiver, so that the receiver will be capable of reproducing the radiated program, based on a geographical region, a specific transmitter, specific announcements, and the like. The radio station recognition can be identified by certain and predetermined modulating frequencies on the auxiliary subcarrier. The amplitude modulation of the auxiliary subcarrier to characterize or recognize a specific station can be in the range of between 20 to 60 Hz, or somewhat higher.
The referenced application Ser. No. 06/319,653, filed Nov. 9, 1981, of which the inventor hereof is a co-inventor, describes a system in which the 57 kHz auxiliary subcarrier is modulated during the duration of an announcement by the announcement recognition, or AR signal with a modulation degree of about 60%, whereas the modulation degree to recognize a certain radio station, or region, that is the RR signal modulation, is then provided at a level of 30%. In the interval between announcements, however, when no announcement is being radiated, the modulation degree of the 57 kHz auxiliary subcarrier by the RR signal frequency is raised to about 60%.
Decreasing the extent of modulation by the RR signal during the period of time that the AR signal is being radiated--with the 60% modulation at its frequency, may cause loss of recognition of the RR signal in the receiver if the geographic distribution between the transmitter and the specific receiver--typically installed in an automotive vehicle--is such that the particular transmitter is being received at only low field signal strength. The modulation degree of 30%, then, may no longer be sufficient to provide the requisite recognition signal strength within the receiver.
It is known to provide receivers with signal-searching apparatus, and such receivers may, additionally, be constructed to respond to the auxiliary subcarrier when modulated with an RR signal. Difficulties may arise in such signal-seeking or scanning or panoramic receivers if the degree of modulation of 30% by the RR signal occurs under conditions of low signal strengths.